Preventing slag deposits on surface to be piered by plasma cutter

ABSTRACT

The present invention is used as a method and an apparatus for effecting piercing start on a member to be To enable a suitable piercing start and good cutting qualities, a slag-deposition preventive (14) is applied to a portion to be pierced of a member (30) to be cut and the vicinity of the same, and a piercing start is thereafter effected, thereby preventing the slag from being deposited on the portion to be pierced and in the vicinity of the same. The apparatus has a reciprocatingly movable carriage (11) on which a plasma torch (12) of a plasma cutter mounted. An applicator (10) for applying the slag-deposition preventive is mounted on the carriage, the applicator being capable of moving together with the plasma torch (12). The apparatus is arranged to easily apply the preventive (30) to a selected portion of the member (30) to be cut.

TECHNICAL FIELD

This invention relates to a method and an apparatus for plasma cuttingwhich are suitable for effecting a piercing start on a member to be cut.

BACKGROUND ART

Generally, there are two methods for cutting a steel plate into variousshapes: one in which cutting is started from an end of the steel plate,and one in which cutting is started from a desirable position in thesteel plate surface. The latter is a piercing start method in which athrough hole is formed in the steel plate at a predetermined position,and in which cutting is started from this through hole (which operationwill hereinafter be referred to as "piercing start"). This piercingstart, in contrast with the former, enables any desired shape in thesteel plate surface as well as a round or triangular shape to be cut.Plasma cutting methods are also based on the above two methods. In suchcutting method, with respect to piercing start in particular, a plasmacutting apparatus such as the one described below is employed and thereare serious problems relating to it. That is, as shown in FIG. 6a,before a hole 311 is formed through a member 30 to be cut (whichoperation is hereinafter referred to as "piercing"), the cut materialmelted by a plasma torch 12 is blown to a portion in the vicinity of thehole 311, attaches thereto, and is further accumulated as slag 32. Asshown in FIG. 6b, the slag 32 interferes with the extreme end of theplasma torch 12 and acts as a cause of damage thereto when the plasmatorch 12 is moved. Further, in the state shown in FIG. 6a, the meltwhich is blown up attaches to the extreme end of the plasma torch 12 sothat the plasma jetting is obstructed, and therefore the melt may causea reduction in cutting quality as well as damage to the plasma torch.The problem of such a drawback is very serious with respect to cuttingof a thick plate by plasma cutting, because the amount of moltenmaterial is increased if the thickness of the member to be cut isincreased. Conventionally, some methods are used to cope with thisproblem, as described below. At the time of piercing, the height of theplasma torch is increased and piercing is performed in this state, andthe height of the plasma torch is thereafter reduced and cutting isperformed, or cutting is started from a high position, thereby avoidingoccurrence of the troublesome condition. Such methods, however, are notsuitable as a means for solving the problem if improvements in qualitywith respect to piercing accuracy and cutting accuracy are required.

A slag-deposition preventive is known. Conventionally, a slag-depositionpreventive is used in such a manner that it is applied to portions of amember to be cut including a cut end of the reverse surface and thevicinity of the same during cutting. It is used for the purpose ofimproving the cutting quality (e.g., appearance) of the reverse surfaceof the cut member by preventing deposition of slag on the portion towhich the preventive is applied. That is, no idea is known whichconnects with a method of applying a slag-deposition preventive to theportion to be pierced to obtain desirable effects.

In view of the above-described problems of the prior art, an object ofthe present invention is to provide a plasma cutting method whichenables a suitable piercing start and to provide a plasma cuttingapparatus suitable for practicing this method.

DISCLOSURE OF THE INVENTION

A plasma cutting method for effecting a piercing start using a plasmacutter in accordance with the present invention comprises applying aslag-deposition preventive to a portion to be pierced of a member to becut and the vicinity of the portion to be pierced, and thereaftereffecting a piercing start, and thereby enables prevention of slagdeposition on the portion to be pierced and the vicinity of the same. Aplasma cutting apparatus suitable for practicing this method comprisesan applicator for applying a slag-deposition preventive mounted on thecarriage, the applicator being capable of moving together with theplasma torch, whereby the preventive is applied to a selected portion ofthe member to be cut.

The process of the plasma cutting method is as described below. Sincethe slag-deposition preventive is previously applied to the portion tobe pierced in the surface of the member to be cut and the vicinity ofthe portion to be pierced, it is difficult for the molten material blownup at the time of piercing to attach to the pierced portion and thevicinity of the same. With respect to the whole surface of the member tobe cut as well, the molten material blown up from the piercing positionis blown far off by the plasma gas supplied from the plasma torch and iscooled while being blown so as to fall down to other positions (that is,not easily deposited).

Since the applicator for applying the slag-deposition preventive ismounted on the reciprocatingly movable carriage moving together with theplasma torch of the plasma cutter, it is easy to apply theslag-deposition preventive to any selected portion of the member to becut as well as the portion to be pierced and the vicinity of the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a, 1b and 1c are diagrams of the comparison between the effectsof an embodiment of the plasma cutting method in accordance with thepresent invention and the prior art in which:

FIG. 1a is a diagram of the compared effects;

FIG. 1b is a diagram of a comparison measure, i.e., a slag depositionheight; and

FIG. 1c is a diagram of another comparison measure, i.e., a slagdeposition range.

FIG. 2 is a schematic diagram of the construction of a first embodimentof a plasma cutting apparatus in accordance with the present invention.

FIG. 3 is a schematic diagram of the construction of a second embodimentof the plasma cutting apparatus in accordance with the presentinvention.

FIG. 4 is a schematic diagram of the construction of a third embodimentof the plasma cutting apparatus in accordance with the presentinvention.

FIGS. 5a and 5b are schematic diagrams of the construction of an X-Ytable on which the plasma cutter in accordance with the presentinvention is mounted.

FIGS. 6a and 6b are diagrams showing the problems of the conventionalplasma cutting method, in which:

FIG. 6a is a diagram of piecing; and

FIG. 6b is a diagram of a movement of the plasma torch.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the plasma cutting method in accordance with thepresent invention will be described below with reference to FIG. 1. Inthis embodiment, a mixture of fine particle graphite and a solventtrichloroethane (commercial name: Unicon Laser Nondross) was used as aslag-deposition preventive.

The performance of this embodiment will be described below. Theconditions for carrying out this embodiment were as follows. The plasmatorch operating current was 80 A, the plasma torch height was 8 mm,oxygen gas was used as plasma gas, and a soft steel plate having athickness of 12 mm was used as a member to be cut. Under theseconditions, plasma cutting was performed 10 times by each of theconventional piercing method using no slag-deposition preventive and themethod of previously applying a slag-deposition preventive to theportion to be pierced and the vicinity of the same in accordance withthe present invention. FIG. 1a shows the results of a comparisontherebetween. This comparison was made with respect to two measures: thedeposition height h (mm), i.e., the maximum height of the mass of slag32 deposited on the portion to be pierced of member 30 to be cut and thevicinity of the same, which was defined as shown in FIG. 1b; and thedistance l (mm) between the piercing center to the farthest end of slag32 continuously extending from the piercing center (hereinafter referredto as "deposition range"), which was defined as shown in FIG. 1c. FIG.1a is a table showing the result of the comparison between averagevalues of piercing performed 10 times by the two methods with respect tothese comparison measures. As shown in this table, the method of thepresent invention limited the deposition height h to about 58% of thedeposition height in the case of the conventional method, and thedeposition range l to about 28% of the corresponding value of theconventional method, thus attaining remarkable effects. For anotherpossible embodiment of the present invention, a slag-depositionpreventive containing a material other than graphite as a mainconstituent may be used. In such a case as well, the same remarkableeffects as the above embodiment can be exhibited as long as thepreventive has a slag-deposition preventing effect.

In sum, it is possible to remarkably reduce the amount of slag depositedon the piercing portion by blowing-up of the molten material duringpiercing, which slag relates the serious problem of the prior art. Thepresent invention therefore ensures the following effects.

(1) Collision of the plasma torch tip and the slag during the movementof the plasma torch can be avoided and the extent of damage to theplasma torch can therefore be remarkably reduced.

(2) The molten material can easily be blown far off by a plasma gas. Itis thereby possible to remarkably reduce the extent of damage to theplasma torch and to improve piercing start and cutting qualities.

(3) By the effects (1) and (2), it is possible to start cutting whilesetting the plasma torch at a smaller height, and to thereby maintaingood cutting qualities.

(4) Since the molten material is blown far off the piercing by theplasma gas, it is possible to maintain the plasma torch at an optimumheight. It is therefore possible to effect a suitable piercing starteven when the thickness of the plate is large.

Next, the overall construction of an ordinary plasma cutting apparatuswill be described below with reference to FIGS. 5a and 5b fordescription of embodiments of the plasma cutting apparatus, andthereafter the first embodiment (FIG. 2), the second embodiment (FIG. 3)and the third embodiment (FIG. 4) will be described.

The plasma cutting apparatus on which a plasma cutter is mounted is, forexample, an X-Y table or robot. Each of the first to third embodimentsdescribed below is an example of the plasma cutting apparatus in which aplasma cutter in accordance with the first, second or third embodimentis mounted on an X-Y table. FIGS. 5a and 5b schematically show the X-Ytable. FIG. 5a is a front view, and FIG. 5b is a side view (in thedirection of A--A of FIG. 5a). Referring to FIG. 5b, an Y shaft 22 isdriven by a ball screw 24 and can be moved as desired in the direction Yalong a rail 25. A reciprocatingly movable carriage 11 on which a plasmatorch is mounted is driven by a ball screw 21 on the Y shaft 22 and canbe moved as desired in the direction X along a rail 23.

In the first embodiment, as shown in FIG. 2, a plasma torch 12 and aslag-deposition preventive applicator 10 are mounted on areciprocatingly movable carriage 11. The slag-deposition preventiveapplicator 10 is also moved with the movement of the reciprocatinglymovable carriage 11. More specifically, the slag-deposition preventiveapplicator 10 is comprised of a tank 13 containing a slag-depositionpreventive 14, a brush 16, and a pipe 15 which connects the tank 13 andthe brush 15 and the which the slag-deposition preventive 14 is suppliedto the brush 15. The operation of this embodiment is as described below.First, the reciprocatingly movable carriage 11 is moved to a piercingposition on a member 30 to be cut. Then, the slag-deposition preventive14 supplied from the tank 13 through the pipe 15 reaches the brush 16,and the slag-deposition preventive 14 is applied to the portion to bepierced and the vicinity of the same by finely moving the plasma torchmount shaft. The position at which the slag-deposition preventive 14 isapplied is not limited to the piercing position alone; it can be appliedat any position selected. Thereafter, the plasma torch 12 is returned tothe per icing position to perform piercing, followed by cutting.

In the second embodiment, as shown in FIG. 3, the slag-depositionpreventive applicator 10 is designed as a spray type. This theslag-deposition preventive applicator 10 is comprised of a tank 13containing a the slag-deposition preventive 14, a spraying nozzle 161,and a pipe 151 which connects the tank 13 and the nozzle 161 with a pumpinterposed therebetween, and through which the slag-depositionpreventive 14 is supplied from the tank 13 to the nozzle 161. Theoperation of this embodiment is as described below. First, thereciprocatingly movable carriage 11 is moved so that the direction ofspraying of the nozzle 161 coincides with a piercing position on themember 30 to be cut. Then, the pump 17 is operated to supply theslag-deposition preventive 14 form the tank 13 to the nozzle 161 throughthe pipe 15, and the slag-deposition preventive 14 is applied to theportion to be pierced and the vicinity of the same by spraying.Thereafter, the plasma torch 12 is returned to the piercing position toperform piercing, followed by cutting. If the arrangement is such thatthe nozzle 161 is inclined relative to the plasma torch 12 asillustrated, the accuracy with which the slag-deposition preventive 14is applied right below the plasma torch 12 or in the vicinity of theright-below position can be improved, and it is possible to remove theabove-described step of returning the plasma torch 12 to the piercingposition after the application of the slag-deposition preventive 14, andto effect piercing while applying the preventive. Further, a pluralityof nozzles 161 may be disposed around the plasma torch 12 to enablewider application.

As described above, the slag-deposition preventive applicators 10 usedin the first and second embodiments are of brush and spray types,respectively. Other types of applicator including a well-known rollertype and a stamp type may be used in place of them.

In the third embodiment, as shown in FIG. 4, the same spray-typeslag-deposition preventive applicator 10 as that of the secondembodiment is mounted on a reciprocatingly movable carriage 11 having apiercing plasma torch 12 and a cutting plasma torch 12b. The operationof this embodiment is as described below. First, the piercing plasmatorch 12a is moved to a piercing position on the member 30 to be cut,and the slag-deposition preventive 14 is applied in a spray coatingmanner by the spray-type slag-deposition preventive applicator. Then,the piercing plasma torch 12a is moved to the piercing position toperform piercing. Thereafter, the cutting plasma torch 12b is moved tothe piercing position to start cutting. By this arrangement, it ispossible to prevent attachment of the molten material to the cuttingplasma torch 12b caused by blowing-up at the time of piercing since thistorch is not used for piecing, as well as to reduce the amount of moltenmaterial attached to the upper surface of the cut member 30.Consequently, it is possible to maintain the cutting accuracy withimproved stability.

The effects of these embodiments will be described below in comparisonwith the conventional plasma cutter. Ordinarily, a working machinehaving a plasma cutter is operated by an operator at a distance from themember to be cut. For this reason, if the plasma cutting method of thepresent invention is applied to the conventional plasma cutter, it isnecessary to previously apply a slag-deposition preventive to a portionto be pierced by a manual operation each time piercing is performed.Accordingly, if cutting is started in the piercing start manner from aplurality of points of the member to be cut, the burden on the operatoris increased, resulting in a deterioration of the productivity. Such aproblem may be solved by covering the whole of the member to be cut.However, to do so requires preparation of an extra amount ofslag-deposition preventive and the operation of removing the unnecessaryslag-deposition preventive from the cut member after cutting, whichreduces the productivity. In contrast, in the embodiments of the presentinvention, the operator can apply the anti-deposition preventive easilyand automatically at a desirable position (piercing position and otherparticular positions) by a remote control operation.

INDUSTRIAL APPLICABILITY

The present invention is used as a method and apparatus for plasmacutting to effect a piecing start on a member to be cut, and isparticularly effective when used a method and apparatus for plasmacutting to prevent deposition of slag on a portion to be pierced and thevicinity of the same.

What is claimed is:
 1. A plasma cutting method for effecting a piercingstart in a member to be cut using a plasma torch, said member having asurface, said surface including a surface portion in which a piercing ofsaid member is to be initiated, said method comprising the stepsof:applying a slag-deposition preventive to said surface portion and tosaid surface in the vicinity of said surface portion; and thereafterutilizing said plasma torch to initiate a piercing in said surfaceportion.
 2. A plasma cutting method according to claim 1, wherein theslag-deposition preventive is sprayed onto said surface portion and ontosaid surface in the vicinity of said surface portion from a position onthe periphery of said plasma torch while said surface portion ispositioned right below said plasma torch.
 3. A method in accordance withclaim 1 wherein said slag-deposition preventive is sprayed onto saidsurface portion by a nozzle directed toward said surface of said memberwhen said member is in position to be cut.
 4. A method in accordancewith claim 1 wherein, during the application of said slag-depositionpreventive to said surface portion in which the piercing is to beinitiated, said plasma torch is positioned so that the center of theplasma torch coincides with said surface portion in which the piercingis to be initiated.
 5. A method in accordance with claim 1 wherein afterthe completion of the application of said slag-deposition preventive tosaid surface portion in which the piercing is to be initiated, saidplasma torch is moved so that the center of said plasma torch coincideswith said surface portion in which the piercing is to be initiated.
 6. Amethod in accordance with claim 1 wherein upon the completion of thepiercing of said member by said plasma torch, said plasma torch isrepositioned away from the piercing position, sand a second plasma torchis then moved to said piercing position and is utilized to cut saidmember, thereby avoiding attachment of molten material to said secondplasma torch at the time of piercing said member at said piercingposition.
 7. A plasma cutting apparatus for effecting a piercing startin a member to be cut using a plasma torch, said member having asurface, said surface including a surface portion in which a piercing ofsaid member is to be initiated, said apparatus comprising areciprocatingly movable carriage, a plasma torch of a plasma cuttermounted on said carriage, and an applicator for applying aslag-deposition preventive to said surface portion in which a piercingof said member is to be initiated, said applicator including a containerof said slag-deposition preventive, said applicator being mounted onsaid carriage, said carriage being capable of moving said applicatortogether with the plasma torch in an X direction and in a Y direction inan X-Y plane on the same side of a member to be cut by said plasmatorch.
 8. A plasma cutting apparatus according to claim 7, wherein saidapplicator has a nozzle for spraying the slag-deposition preventive ontosaid surface portion.
 9. A plasma cutting apparatus according to claim7, wherein said applicator has a plurality of nozzles for spraying theslag-deposition preventive onto said surface portion and onto saidsurface in the vicinity of said surface portion.
 10. A plasma cuttingapparatus according to claim 7, wherein said applicator furthercomprises a nozzle for spraying the slag-deposition preventive onto saidsurface portion, and a pump for supplying said slag-depositionpreventive from said container to said nozzle.
 11. A plasma cuttingapparatus according to claim 10, wherein said nozzle is inclinedrelative to the plasma torch so that the slag-deposition preventive isapplied onto said surface portion, said surface portion being rightbelow the plasma torch.
 12. A plasma cutting apparatus according toclaim 7, wherein said applicator further comprises a brush for applyingthe slag-deposition preventive onto said surface portion, and a pipe forsupplying said slag-deposition preventive from said container to saidbrush.
 13. A method of cutting a member with a plasma torch utilizing apiercing start, said member having a surface, said surface including asurface portion in which a piercing of said member is to be initiated,said method comprising the steps of:moving a reciprocating base, onwhich a plasma torch and an applicator of slag-deposition preventive aremounted, over said surface of said member so that the pattern of anysaid slag-deposition preventive emitted from said applicator onto saidsurface of said member at least coincides with the piercing startposition on said surface of said member; causing said applicator toapply said slag-deposition preventive onto said surface of said memberat least at said piercing start position to provide a coating of saidslag-deposition preventive on said surface of said member at least atsaid piercing start position on said surface of said member; positioningsaid plasma torch so that it coincides with the piercing start positionon said surface of said member; and utilizing the thus positioned plasmatorch to initiate a piercing of said member at said piercing startposition on said surface of said member to cut a hole through saidmember at the thus coated piercing start position on said surface ofsaid member.
 14. A method in accordance with claim 13, wherein saidapplicator comprises a nozzle directed toward said surface of saidmember when said member is in position to be cut, a source ofslag-deposition preventive, and a pipe for supplying saidslag-deposition preventive from said source to said nozzle.
 15. A methodin accordance with claim 14, wherein said plasma torch is positioned sothat the center of the plasma torch coincides with the piercing startposition on said surface of said member during the application of saidcoating of said slag-deposition preventive at said piercing startposition on said surface of said member.
 16. A method in accordance withclaim 13, wherein after the completion of the application of saidcoating of said slag-deposition preventive at said piercing startposition on said surface of said member, said reciprocating base isrepositioned so that the center of said plasma torch coincides with thepiercing start position on said surface of said member.
 17. A method inaccordance with claim 15 wherein after the completion of the applicationof said coating of said slag-deposition preventive at said piercingstart position on said surface of said member, said reciprocating baseis repositioned so that the center of said plasma torch coincides withthe piercing start position on said surface of said member.
 18. A methodin accordance with claim 17 wherein upon the completion of the piercingof said member at said piercing start position to cut a hole throughsaid member, said plasma torch is repositioned away from said piercingstart position, and a second plasma torch is then moved to said piercingstart position and is utilized to cut said member, thereby avoidingattachment of molten material to said second plasma torch at the time ofpiercing said member at said piercing start position.
 19. A method inaccordance with claim 13 wherein said plasma torch is positioned so thatthe center of the plasma torch coincides with the piercing startposition on said surface of said member during the application of saidcoating of said slag-deposition preventive at said piercing startposition on said surface of said member.
 20. A method in accordance withclaim 13 wherein upon the completion of the piercing of said member atsaid piercing start position to cut a hole through said member, saidplasma torch is repositioned away from said piercing start position, anda second plasma torch is the moved to said piercing start position andis utilized to cut said member, thereby avoiding attachment of moltenmaterial to said second plasma torch at the time of piercing said memberat said piercing start position.